What happens to everyone's paperwork when a domestic flight has to land in another country?

There is an established approach, and the first thing to understand is that the default is nobody enters the country at all.

An aircraft on the ground with its passengers aboard has not, in immigration terms, admitted anyone. If the diversion is short — refuelling, waiting out weather, a medical case being taken off — the ordinary outcome is that everyone stays on the aircraft or in a sterile transit area, and the question of documents never formally arises.

That covers the large majority of diversions. Documents only become an issue when passengers have to be taken off and cannot be put back on within a reasonable time: a technical fault requiring a replacement aircraft, a crew running out of legal duty hours, an airport closing overnight.

There is a well-known mass example: when a large amount of North American airspace was closed at short notice in 2001, a great many aircraft were sent to Canadian airports, and thousands of people ended up in small towns for days with whatever documents they happened to have.

It was handled, and it was handled by improvisation on top of exactly the discretionary framework described above. Which is roughly the answer to your question — there is a procedure for the common short case, and beyond that there is discretion plus a lot of phone calls.

The practical takeaway that comes up every time this is discussed: on domestic routes that pass near a border, carrying a passport costs nothing and occasionally saves you a very long day.

It does not change the group outcome, but it does change how quickly you personally get processed and whether you can be rebooked onward independently rather than waiting for the group to be cleared together.

When it does go past that point, the mechanism is that authorities have discretion and they generally use it, because the alternative is worse for everybody.

What happens in practice:

• Passengers are processed as a group rather than individually, often in a separate area, with the airline handling the paperwork rather than each person presenting a case.
• Entry may be granted on a temporary or exceptional basis, sometimes with the group kept together and accommodated under supervision.
• Documents that would normally be insufficient get accepted, because the relevant question has shifted from "is this person entitled to visit" to "how do we get these people onward".

The underlying logic is that nobody chose to be there, everyone has an obvious onward destination, and the airline is contractually responsible for moving them. That is a very different risk picture from an ordinary arrival, and border authorities treat it as one.